Method for manufacturing polyimide asymmetric membrane, and polyimide asymmetric membrane

ABSTRACT

A process of producing an asymmetric membrane of multicomponent polyimide. The process includes the steps of (1) preparing a multicomponent polyimide blend solution by mixing a polyimide component A having a number-averaged polymerization index N A  and a polyimide component B having a number-averaged polymerization index N B , wherein N A  and N B  satisfies equation 1:
 
2.35× N   A   −2.09   &lt;N   B &lt;450× N   A   −1.12   1
 
(2) subjecting the multicomponent polyimide blend solution to further polymerization and imidation reaction, and (3) causing a phase inversion in the resulting multicomponent polyimide blend solution to form an asymmetric membrane. The polyimide component A is raw materials of polyimide A containing a fluorine atom in the chemical structure thereof and/or a polymerization and imidation reaction product of the raw materials. The polyimide component B is raw materials of polyimide B and/or a polymerization and imidation reaction product of the raw materials.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process of producing a polyimide asymmetricmembrane having a dense layer and a porous layer. More particularly, itrelates to a process of producing a polyimide asymmetric membrane madeof multi-component polyimide containing a fluorine-containing polyimideand having a controlled composition of the fluorine-containing polyimidein its dense layer. The process of the invention provides a practicalhigh-performance gas separation membrane.

2. Description of the Related Art

Gas separation membranes are made use of in industrial gas separationprocessing. Above all, gas separation membranes made of polyimide withhigh permselectivity (permeance ratio) are used. Generally speaking,polyimide has high permselectivity (permeance ratio) but lowpermeability (permeability coefficient). Hence, a gas separationmembrane made of polyimide has an asymmetric structure composed of aporous layer primarily performing a mechanical supporting function and adense layer primarily performing a separation function, with thethickness of the dense layer, where permeate gas undergoes permeationresistance, reduced so as to secure a gas permeation rate.

A gas separation membrane for practical use is required to have not onlygas permeation characteristics including gas permselectivity andpermeation rate but other characteristics such as mechanical strength.In the case of polyimide derived solely from one tetracarboxylic acidcomponent and one diamine component (i.e., homopolyimide), thesecharacteristics are determined by the combination of the tetracarboxylicacid component and the diamine component. In order to realize a gasseparation membrane fulfilling these requirements for practical use,studies of gas separation membranes have been directed to use ofcopolyimide obtained by replacing part of the tetracarboxylic acidcomponent and/or the diamine component with other tetracarboxylic acidcomponent and/or other diamine component. Characteristics of gasseparation membranes made of such copolyimide depend on the compositionof two or more tetracarboxylic acid components and/or two or morediamine components. Through the studies, polyimides prepared using afluorine-containing tetracarboxylic acid component or afluorine-containing diamine component have often been used for thepurpose of improving gas permeation characteristics, particularlypermeation rate.

In general, nevertheless, an asymmetric membrane formed of a polyimidewith excellent gas permeation characteristics, such as afluorine-containing polyimide, has insufficient mechanical strength,while an asymmetric membrane formed of a polyimide with high mechanicalstrength exhibits insufficient gas permeation characteristics.

JP-A-6-269650 discloses a composite gas separation membrane having alaminate structure comprising (a) a porous polyacrylonitrile structuralsupport material, (b) a gutter layer comprising a crosslinked polarphenyl-containing-organopolysiloxane material, and (c) an ultrathinselective membrane layer comprising a specific fluorine-containingpolyimide.

JP-A-8-52332 discloses a composite gas separation membrane comprising analiphatic porous polyimide supporting layer and a fluorine-containingpolyimide thin layer laminated thereon.

Making such a composite membrane involves forming a uniform thin layeron a porous layer. However, it is not easy to uniformly form a thinlayer on a porous layer. In fact, it is not easy even with the processestaught in the above references to obtain a high performance gasseparation membrane.

Japanese Patent Application No. 2003-24755 discloses a process ofproducing an asymmetric hollow fiber separation membrane by phaseinversion method using a polymer blend solution containing two kinds ofpolyimides. The reference does not mention production of an asymmetricmembrane using a blend solution containing a copolymer having“blockness” that is obtained by preparing a blend solution containingpolyimide components having specific polymerization indexes and furthersubjecting the blend solution to polymerization and imidation.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a process of producinga polyimide asymmetric membrane comprising multicomponent polyimidecontaining a fluorine-containing polyimide and having a controlledcomposition of the fluorine-containing polyimide in a dense layerthereof.

Another object of the present invention is to provide a polyimideasymmetric membrane containing a fluorine-containing polyimide in adense layer thereof in a suitably controlled composition.

The present invention relates to a process of producing an asymmetricmembrane of multi-component polyimide. The process includes (1)preparing a multi-component polyimide blend solution by mixing apolyimide component A and a polyimide component B, the polyimidecomponent A being raw materials of a polyimide A containing a fluorineatom in the chemical structure thereof and/or a polymerization andimidation reaction product of the raw materials, the polyimide componentB being raw materials of a polyimide B and/or a polymerization andimidation reaction product of the raw materials, the number-averagedpolymerization index of the polyimide component A taken as N_(A) and thenumber-averaged polymerization index of the polyimide component B takenas N_(B) satisfying equation 1:2.35×N _(A) ^(−2.09) <N _(B)<450×N _(A) ^(−1.12)  1(2) subjecting the multi-component polyimide blend solution to furtherpolymerization and imidation reaction, and (3) causing a phase inversionin the resulting multi-component polyimide blend solution to form anasymmetric membrane.

The present invention also relates to a polyimide asymmetric membranehaving a dense layer and a porous layer. The polyimide asymmetricmembrane comprises a multi-component polyimide containing afluorine-containing polyimide. The ratio of the dense layer's fluorineatom concentration (Φ_(s)), measured by X-ray photoelectron spectroscopy(XPS), to the overall average fluorine atom concentration (f) of themembrane, Φ_(s)/f, ranges from 1.1 to 1.8.

The present invention also relates to a method of separating andrecovering at least one kind of gas from a gas mixture. The methodcomprises feeding the gas mixture to a feed side of the polyimideasymmetric gas separation membrane of the present invention andselectively permeating at least one component of gas of the gas mixturethrough the gas separation membrane to a permeate side.

The present invention provides a polyimide asymmetric membrane having adense layer and a porous layer, comprising a multi-component polyimidecontaining a fluorine-containing polyimide, with the fluorine-containingpolyimide content in the dense layer suitably controlled.

The polyimide asymmetric membrane of the invention is suitable as apractical high-performance gas separation membrane with which separationbetween hydrogen gas and a hydrocarbon gas, e.g., methane gas,separation between hydrogen gas and nitrogen gas, separation betweenhelium gas and nitrogen gas, separation between carbonic acid gas andmethane gas, separation between oxygen gas and nitrogen gas, and thelike can be accomplished advantageously.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents a TEM image of a cross section of a film in whichmacrophase-separation has occurred. The film was obtained by casting apolyimide blend solution of two high-molecular-weight polyimides(Comparative Example 5) on a glass plate, followed by drying.

FIG. 2 presents a TEM image of a cross section of a film in whichmacrophase-separation has not occurred. The film was obtained by castinga multi-component polyimide blend solution of the invention (Example 4)on a glass plate, followed by drying.

FIG. 3 shows a TEM image taken of a cross section of another film inwhich macrophase-separation has occurred. The film was obtained bycasting a blend solution of two high-molecular-weight polyimides on aglass plate, followed by drying. The TEM image gives betterunderstanding of macrophase-separated structure near the film surface.

FIG. 4 is a graph illustrative of the combination range of N_(A) andN_(B) in the invention.

FIG. 5 is the results of dSIMS analysis for fluorine in the thicknessdirection of a film obtained by casting a multi-component polyimideblend solution of the invention (Example 4) on a glass substrate,followed by drying.

FIG. 6 is the results of dSIMS analysis for fluorine in the thicknessdirection of a film obtained by casting a polyimide solution prepared ina usual manner (Comparative Example 2) on a glass substrate, followed bydrying.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The polyimide asymmetric membrane of the present invention will bedescribed with particular reference to its application as ahigh-performance gas separation membrane, but it should be noted thatthe application of the polyimide asymmetric membrane of the invention isnot limited thereto. The terminology “polyimide component” as usedherein means a component comprising raw materials of polyimide (anunreacted tetracarboxylic acid component and an unreacted diaminecomponent) and/or a polymerization and imidation reaction product of theraw materials. The terminology “polymerization and imidation reactionproduct” as used herein does not necessarily means a polymer having ahigh polymerization index but includes a monomer and an oligomer havinga low polymerization index which are produced in the initial stage ofthe polymerization and imidation reaction. Therefore, the polymerizationand imidation reaction product is a mixture of a monomer (i.e., anequimolecular imidation product between a tetracarboxylic acid componentand a diamine component) and/or a polymer (i.e., an imidation product ofmore than 2 molecules, in total, of a tetracarboxylic acid component anda diamine component).

The polymerization index of a polymerization and imidation reactionproduct, as referred to in the invention, is defined to be the number ofrepeating units in a polyimide present in the product. A monomer has apolymerization index of 1, and a polymer has a polymerization index >1.On the other hand, the polymerization index of each raw material ofpolyimide, having no repeating unit, is defined to be 0.5. Anumber-averaged polymerization index is calculated on the basis of theabove-defined polymerization indexes.

The polyimide component A comprises raw materials of a polyimide A(i.e., an unreacted tetracarboxylic acid component and an unreacteddiamine component) and/or a polymerization and imidation reactionproduct of the raw materials. The polyimide component B comprises rawmaterials of polyimide B (i.e., an unreacted tetracarboxylic acidcomponent and an unreacted diamine component) and/or a polymerizationand imidation reaction product of the raw materials.

If the polyimide components A and B in both of which the tetracarboxylicacid component and the diamine component are in their unreacted state(both having a polymerization index of 0.5) are mixed and subjected topolymerization and imidation reaction, there is produced a polyimidemainly comprising a random copolymer in which the polyimide components Aand B are bonded with considerable randomness. When subjected to phaseinversion method, such a polyimide is capable of forming an asymmetricmembrane composed of a dense layer and a porous layer but incapable ofpreferentially segregating a fluorine-containing polyimide to the denselayer, namely incapable of providing a gas separation membraneexhibiting excellent gas permeation characteristics and satisfactorymechanical characteristics. The reason is due to the conflictingrelationship: polyimide with high gas permeation characteristics haspoor mechanical strength, whilst polyimide with satisfactory mechanicalstrength has poor gas permeation characteristics.

If the polyimide component A and the polyimide component B areseparately polymerized and imidated, and the resulting polyimides A andB both having a high polymerization index are blended, it is usuallyhard to prepare a uniform solution of the blend. A uniform solution ofthe blend could be prepared and remain nearly uniform for a very shorttime, but it is not easy to maintain the uniformity for a long timeenough to carry out phase inversion to produce an asymmetric membrane ina stable manner. Where a blend solution containing polyimides havinghigh polymerization indexes is phase inverted, the solution undergoesrapid progress of macrophase-separation induced by repulsive interactionbetween the different polyimides due to differences, even if slightdifferences present, in chemical properties. The terminology“macrophase-separation” denotes phase separation of different componentsof polyimides resulting in forming a macrophase-separated structurecontaining domains of 0.1 μm or greater, not infrequently of 1 μm orgreater, in size comprising different components. A transmissionelectron microscope (TEM) image of an example of such amacrophase-separated structure is presented in FIG. 1. Another exampleis shown in FIG. 3. Clearly distinguished domains of differentcomponents in a macrophase-separated structure is observed in the TEMimages. If macrophase-separation occurs, the dense layer will sufferfrom serious disturbances, resulting in a failure to provide anasymmetric membrane having good separation capabilities. An asymmetricmembrane prepared by a dry/wet spinning process often has a thickness ofabout 1 to 1000 nm in its dense layer. In case of occurrence ofmacrophase-separation, the dense layer will have polyimide A-richdomains and polyimide B-rich domains, which is demonstrated in FIG. 3.In other words, when viewed in an in-plane direction, the dense layer ismacroscopically non-uniform, being composed of domains of differentcomponent. As a result, an asymmetric membrane with satisfactoryseparation capabilities cannot be obtained.

The present invention provides a process of producing an asymmetricmembrane characterized in that a solution of a multi-component polyimidehaving a prescribed polymerization index and containing a blockcopolymer is prepared and that the multi-component polyimide blendsolution is subjected to phase inversion. When the multi-componentpolyimide blend solution of the invention is used in a phase inversionprocess, phase separation which can be seen as microphase-separationproceeds without being accompanied by macrophase-separation. A TEM imagerepresenting an example of such a microphase-separated structure isshown in FIG. 2, in which no phase separated structure containingdomains of different components (i.e., macrophase separation) isobserved. Although it is considered that there are fine domains of aboutseveral nanometers to about 0.1 μm, the boundaries of the domains areunclear when seen as a whole, presenting a structure containing lots ofvague regions where different polyimides are not completelyphase-separated. In the pathway of this phase separation, there isformed a multi-component polyimide layer containing a higher fraction offluorine-containing polyimide in the dense layer when seen in across-sectional direction of the membrane (a direction perpendicular tothe surface of the membrane) as described later. Involvement ofmacroscopic disturbances in polyimide composition in an in-planedirection of the membrane (a direction parallel to the surface of themembrane) is averted. That is, the present invention provides a processof producing an improved multi-component polyimide asymmetric membranein which the dense layer and the porous layer are different in chemicaland physical properties with no variations nor reductions in separationcapabilities that would accompany the progress of macrophase-separation.This can be achieved by causing a microphase-separation betweendifferent polyimides in a controlled manner so as to prevent the phaseseparation from attaining macrophase-separation.

The process of producing a multi-component polyimide asymmetric membraneaccording to the present invention includes the following steps (1) to(3), in which a “polyimide component A” is raw materials of a polyimideA containing a fluorine atom in the chemical structure thereof and/or apolymerization and imidation reaction product of the raw materials; a“polyimide component B” is raw materials of a polyimide B and/or apolymerization and imidation reaction product of the raw materials; thenumber-averaged polymerization index of the polyimide component A istaken as N_(A); and the number-averaged polymerization index of thepolyimide component B is taken as N_(B).

Step 1: A polyimide component A and a polyimide component B are mixed toprepare a multi-component polyimide blend solution, the polyimidecomponent A and the polyimide component B being so combined as tosatisfy equation 1:2.35×N _(A) ^(−2.09) <N _(B)<450×N _(A) ^(−1.12)  (1)Step 2: The multicomponent polyimide blend solution is subjected tofurther polymerization and imidation reaction.Step 3: An asymmetric membrane is formed by a phase inversion methodusing the resulting multi-component polyimide blend solution.

The polyimide A containing a fluorine atom in the chemical structurethereof is a polyimide derived from raw materials, i.e., atetracarboxylic acid component and a diamine component, at least one ofwhich contains a fluorine atom.

Suitable raw materials for polyimide A are those producing polyimide Awith high gas permeation rate and high gas selectivity. Particularlysuitable are those producing a polyimide A having, in the form of auniform film, a helium gas permeability coefficient (P_(He)) of 5×10⁻¹⁰cm³(STP)·cm/cm²·sec·cmHg or greater and a helium to nitrogen gaspermeance ratio (P_(He)/P_(N2)) of 20 or greater as measured at 80° C.,preferably a helium gas permeability coefficient (P_(He)) of 2.5×10⁻⁹cm³(STP)·cm/cm²·sec·cmHg or greater and a helium to nitrogen gaspermeance ratio (P_(He)/P_(N2)) of 20 or greater as measured at 80° C.,more preferably a helium gas permeability coefficient (P_(He)) of 3×10⁻⁹cm³(STP)·cm/cm²·sec·cmHg or greater and a helium to nitrogen gaspermeance ratio (P_(He)/P_(N2)) of 30 or greater as measured at 80° C.Containing fluorine, the polyimide A exhibits higher solubility invarious solvents commonly used in a phase inversion process and havingsmaller surface free energy as compared with polyimide containing nofluorine.

A lower P_(He) or a lower P_(He)/P_(N2) than the respective recitedranges results in insufficient gas selectivity (permeance ratio) andpermeation rate of the resulting asymmetric gas separation membrane. Theabove-recited ranges are therefore appropriate.

Fluorine-containing tetracarboxylic acid components for making apolyimide A include, but are not limited to,2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane,2,2′-bis(trifluoromethyl)-4,4′,5,5′-biphenyltetracarboxylic acid,4,4′-(hexafluorotrimethylene)diphthalic acid,4,4′-(octafluorotetramethylene)diphthalic acid, and their dianhydrideand esters. Preferred of them are2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane and its dianhydride(hereinafter sometimes abbreviated as 6FDA) and esters.

Fluorine-containing diamine components for making a polyimide A include,but are not limited to, 2,2-bis(4-aminophenyl)hexafluoropropane,2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl,2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, and2-trifluoromethyl-p-phenylenediamine.

These fluorine-containing raw materials may be used either individuallyor as a mixture of two or more thereof or in combination with afluorine-free monomer component. It is preferred that either thetetracarboxylic acid component or the diamine component contains afluorine-containing raw material in a major fraction (i.e., a molarcomposition of 50 mol % or more, usually 55 mol % or more).

Where a fluorine-containing tetracarboxylic acid component is a majortetracarboxylic acid component to make polyimide A, the diaminecomponent that can be used in combination includes aromatic diamines,such as p-phenylenediamine, m-phenylenediamine (hereinafter sometimesabbreviated as MPD), 4,4′-diaminodiphenyl ether (hereinafter sometimesabbreviated as DADE), 4,4′-diaminodiphenylmethane,3,3′-dimethyl-4,4′-diaminodiphenylmethane,3,3′,5,5′-tetramethyl-4,4′-diaminodiphenylmethane,3,3′-dichloro-4,4′-diaminodiphenylmethane,dimethyl-3,7-diaminodibenzothiophene 5,5-dioxide (hereinafter sometimesabbreviated as TSN; TSN is usually available in the form of a mixturehaving 2,8-dimethyl-3,7-diaminodibenzothiophene 5,5-dioxide as a maincomponent and containing isomers with a methyl group(s) bonded atdifferent positions, e.g., 2,6-dimethyl-3,7-diaminodibenzothiophene5,5-dioxide and 4,6-dimethyl-3,7-diaminodibenzothiophene 5,5-dioxide),2,2-bis[4-(4-aminophenoxy)phenyl]propane,3,3′-dihydroxy-4,4′-diaminodiphenyl,3,3′-dicarboxy-4,4′-diaminodiphenyl,3,3′-dicarboxy-4,4′-diaminodiphenylmethane,3,3′,5,5′-tetrachloro-4,4′-diaminodiphenyl, diaminonaphthalene,2,4-dimethyl-m-phenylenediamine, 3,5-diaminobenzoic acid (hereinaftersometimes abbreviated as DABA), and 3,3′-diaminodiphenylsulfone(hereinafter sometimes abbreviated as MASN). Where 6FDA or itsderivative is used as a major tetracarboxylic acid component,particularly preferred diamines to be combined with of theabove-enumerated aromatic diamines are those having an amino group atthe meta-position, such as DABA, MASN, and MPD.

Tetracarboxylic acid components that can be combined with afluorine-containing diamine component for making polyimide A include,but are not limited to, pyromellitic acid, benzophenonetetracarboxylicacid, naphthalenetetracarboxylic acid, bis(dicarboxyphenyl) ether,bis(dicarboxyphenyl) sulfone, 2,2-bis(dicarboxyphenyl)propane,2,3,3′,4′-biphenyltetracarboxylic acid,2,2′,3,3′-biphenyltetracarboxylic acid,3,3′,4,4′-biphenyltetracarboxylic acid, and their anhydride and esters.Particularly preferred of them is 3,3′,4,4′-biphenyltetracarboxylic aciddianhydride (hereinafter sometimes abbreviated as s-BPDA).

Suitable raw materials for a polyimide B are such that a film obtainedfrom the resulting polyimide B has a tensile strength of 100 MPa ormore, preferably 150 MPa or more, and a tensile elongation at break of10% or more, preferably 15% or more. With the tensile strength less than100 MPa or the tensile elongation at break less than 10%, an asymmetricmembrane obtained using such polyimide has insufficient mechanicalstrength and ductility for practical use, failing to be fabricated intogas separation modules or failing to be suited to applications usinghigh pressure gas. Therefore, the above-recited ranges are appropriate.

Since polyimide having a fluorine atom in the chemical structure thereofhas relatively low mechanical strength, it is preferred for both themonomer components providing polyimide B, i.e., a tetracarboxylic acidcomponent and a diamine component, not to contain a fluorine-containingcomponent in a major fraction. It is more preferred for both the monomercomponents to be free of fluorine.

Tetracarboxylic acid components of a polyimide B include, but are notlimited to, pyromellitic acid, benzophenonetetracarboxylic acid,naphthalenetetracarboxylic acid, bis(dicarboxyphenyl) ether,bis(dicarboxyphenyl) sulfone, 2,2-bis(dicarboxyphenyl)propane,2,3,3′,4′-biphenyltetracarboxylic acid,2,2′,3,3′-biphenyltetracarboxylic acid,3,3′,4,4′-biphenyltetracarboxylic acid, and their anhydride and esters.Particularly preferred of them is 3,3′,4,4′-biphenyltetracarboxylic aciddianhydride.

These tetracarboxylic acid components can be used either individually oras a mixture of two or more thereof or in combination with a smallamount of a fluorine-containing tetracarboxylic acid component. Forexample, it is acceptable that one mole of s-BPDA is combined with notmore than 0.3 moles of 6FDA.

Suitable diamine components of a polyimide B include those listed aboveas diamine components that can be combined with 6FDA as a majortetracarboxylic acid component of polyimide A.

In step 1 of the process of the invention, a multi-component polyimideblend solution is prepared by mixing (i) a polyimide component A havinga number-averaged polymerization index N_(A) and comprising rawmaterials of a polyimide A containing a fluorine atom in the chemicalstructure thereof and/or a polymerization and imidation reaction productof the raw materials and (ii) a polyimide component B having anumber-averaged polymerization index N_(B) and comprising raw materialsof a polyimide B and/or a polymerization and imidation reaction productof the raw materials. The N_(A) and N_(B) satisfy equation 1. In FIG. 4,the range of the N_(A)/N_(B) combination that satisfies equation 1 isgraphically shown as a shaded region. The polymerization index of thepolyimide raw materials (i.e., unreacted tetracarboxylic acidcomponent(s) and unreacted diamine component(s)) being defined to be0.5, N_(A) and N_(B) are at least 0.5.

In step 2, the multicomponent polyimide blend solution is subjected tofurther polymerization and imidation reaction to give a mixturecontaining the polyimide component A and the polyimide component B bothhaving been further polymerized and imidated. The result is amulti-component polyimide blend solution containing at least a polymerfrom the polyimide A component and a polymer from the polyimide Bcomponent. The blend solution additionally contains a di- or multi-blockcopolymer having the polyimide component A and the polyimide component Bbonded to each other at their ends and has acquired an appropriatepolymerization index.

The term “diblock copolymer” denotes a copolymer consisting of one blockcomposed of a polyimide component A and one block composed of apolyimide component B joined end-to-end. The term “multi-blockcopolymer” denotes a copolymer consisting of the diblock copolymer andat least one block of either kind bonded to one or both ends of thediblock copolymer. The di- or multi-block copolymer can contain a chainof blocks of a polyimide component A or a chain of blocks of a polyimidecomponent B.

The description goes into further detail by referring to FIG. 4.

If polyimide components A and B whose N_(A) and N_(B) are in region A inthe graph of FIG. 4 are mixed in step 1, and the resultingmulti-component polyimide blend solution is polymerized and imidated instep 2, neither blocks consisting solely of the polyimide component Anor blocks consisting solely of the polyimide component B are formed,and the result is a copolymer having the polyimide components A and Bdistributed with high randomness.

If polyimide components A and B whose N_(A) and N_(B) are in region B inthe graph of FIG. 4 are mixed in step 1, and the resultingmulti-component polyimide blend solution is polymerized and imidated instep 2, a multi-component polyimide blend solution containing a blockcopolymer could be obtained. However, because of too high thepolymerization index of the block copolymer, strong repulsiveinteractions between the polyimide blocks easily result inmacrophase-separation. Therefore, N_(A) and N_(B) combinations inregions A and B in FIG. 4 fail to provide the asymmetric membrane of thepresent invention.

Within the N_(A) and N_(B) combination range satisfying equation 1 (theshaded region in the graph of FIG. 4), there is obtained amulti-component polyimide blend solution containing at least a polymerfrom the polyimide A component, a polymer from the polyimide B componentand, in addition, a di- or multi-block copolymer having the one blockcomposed of polyimide component A and the one block composed ofpolyimide component B bonded end-to-end and, as a whole, having anappropriate polymerization index. It is possible with the resultingmulti-component polyimide to achieve controlled phase separation, whatwe may call microphase-separation, while inhibitingmacrophase-separation that might be caused by repulsive interactions.

Step 1 is the step in which a polyimide component A comprising rawmaterials of a polyimide A containing a fluorine atom in the chemicalstructure thereof and/or a polymerization and imidation reaction productof the raw materials and a polyimide component B comprising rawmaterials of a polyimide B and/or a polymerization and imidationreaction product of the raw materials, the number-averagedpolymerization index of the polyimide component A (N_(A)) and thenumber-averaged polymerization index of the polyimide component B(N_(B)) satisfying equation 1, are mixed to prepare a multicomponentpolyimide blend solution. The manner of effecting step 1 is notparticularly limited as long as a multi-component polyimide blendsolution is obtained. For example, raw materials of a polyimide A andraw materials of a polyimide B are separately prepared, if necessary bypolymerization and imidation reaction. They are uniformly mixed toobtain a multi-component polyimide blend solution. When either one ofthe polyimide components is a mixture of raw materials (i.e., anunreacted tetracarboxylic acid component and an unreacted diaminecomponent), the raw materials of the other polyimide component can bepolymerized and imidated to prepare a polyimide solution having aprescribed number-averaged polymerization index. To the polyimidesolution are added the unreacted tetracarboxylic acid component and theunreacted diamine component as the first mentioned polyimide componentto give a multicomponent polyimide blend solution. Considering that apolyimide B having a higher polymerization index is more advantageous toimprove the mechanical strength of an asymmetric membrane, it isadvantageous that the raw materials of a polyimide B are polymerized andimidated in a polar solvent to prepare a polyimide B with an appropriatepolymerization index, into which the raw materials of a polyimide A aremixed to prepare a multi-component polyimide blend solution in step 1.

Polymerization and imidation reaction for obtaining polyimide isdescribed below. Polymerization and imidation reaction is carried outconveniently by allowing a tetracarboxylic acid component and a diaminecomponent to react at a predetermined ratio in a polar solvent at 120°C. or higher, preferably 160° C. or higher, and not higher than theboiling point of the solvent, whereby polyamic acid is formed, followedby dehydration and ring closure to form an imide group. In order toachieve a prescribed polymerization index, the reaction temperature maybe lowered than the recited range. Because a residual amic acid groupcan undergo exchange reaction to impair the blockness of polyimide, thepolymerization and imidation reaction is preferably carried out toachieve an imidation ratio of at least 50%, more preferably untilimidation substantially completes.

The polymerization and imidation reaction between a tetracarboxylic acidcomponent and a diamine component at a ratio close to 1 results insynthesis of polyimide with a relatively high molecular weight (a highnumber-averaged polymerization index). Hence, when a polyimide having arelatively high molecular weight from the beginning is desired, it ispreferred to cause a tetracarboxylic acid component and a diaminecomponent to react at a molar ratio of 1:0.95 to 0.99 or 1:1.005 to1.05, more preferably 1:0.98 to 0.995 or 1:1.005 to 1.02, to obtain apolyimide component having a relatively high molecular weight.

In the case of, for example, 6FDA as a tetracarboxylic acid componentand TSN as a diamine component, dehydration and ring closure reactionusing 1.02 moles of TSN per mole of 6FDA at 190° C. for 30 hours resultsin synthesis of a polyimide having a number-averaged molecular weight ofabout 15000 to 25000 (corresponding to a number-averaged polymerizationindex of about 20 to 40); and dehydration and ring closure reactionusing 1.005 moles of TSN per mole of 6FDA at 190° C. for 30 hoursresults in synthesis of a polyimide having a number-averaged molecularweight of about 30000 to 40000 (corresponding to a number-averagedpolymerization index of about 40 to 60).

In another example, where 6FDA and DABA are used as a tetracarboxylicacid component and a diamine component, respectively, dehydration andring closure reaction using 1.02 moles of DABA per mole of 6FDA at 190°C. for 30 hours results in synthesis of a polyimide having anumber-averaged molecular weight of about 15000 to 25000 (correspondingto a number-averaged polymerization index of about 25 to 45); anddehydration and ring closure reaction using 1.005 moles of DABA per moleof 6FDA at 190° C. for 30 hours results in synthesis of polyimide havinga number-averaged molecular weight of about 40000 to 50000(corresponding to a number-averaged polymerization index of about 70 to90).

On the other hand, reaction between 1 mol of a tetracarboxylic acidcomponent and 0.98 mol or less or 1.02 mol or more of a diaminecomponent results in formation of a polyimide component tailored to havea relatively low molecular weight (a small number-averagedpolymerization index).

The multi-component polyimide blend solution obtained in step 1preferably has a total diamine component to total tetracarboxylic acidcomponent molar ratio (total number of moles of a diaminecomponents)/total number of moles of a tetracarboxylic acidcomponent(s)) ranging from 0.95 to 0.99 or from 1.01 to 1.05, morepreferably from 0.96 to 0.99 or from 1.015 to 1.04. The recited totaldiamine component to total tetracarboxylic acid component molar ratio isadvantageous for obtaining a multi-component polyimide blend solutionwith an appropriate number-averaged molecular weight or solutionviscosity in step 2.

Step 2 is the step of subjecting the multi-component polyimide blendsolution obtained in step 1, which contains the polyimide A componentand polyimide B component whose N_(A) and N_(B) satisfy equation 1, tofurther polymerization and imidation reaction to prepare a blendsolution of a multi-component polyimide containing at least a polymerfrom the polyimide A component, a polymer from the polyimide B componentand, in addition, a di- or multi-block copolymer having the one blockcomposed of polyimide component A and the one block composed ofpolyimide component B bonded end-to-end and, as a whole, having anappropriate polymerization index.

Step 2 consists in subjecting the multi-component polyimide blendsolution obtained in step 1 to further polymerization and imidationreaction. The above-described method for polymerization and imidationcan be adopted as appropriate.

A polar solvent capable of uniformly dissolving multi-componentpolyimide is used in the multi-component polyimide blend solutionsprepared in steps 1 and 2. The expression “uniformly dissolving” as usedherein means that the solvent is capable of providing a solution freefrom macrophase-separated domains large enough to scatter visible lightand free from apparently obvious turbidity. The solution may containmicrophase-separated domains of sizes not so large as to cause visiblelight scattering. The solution is not indispensably required to beuniform on the molecular chain level.

If a solvent used is such that the multi-component polyimide solutiondevelops apparently obvious turbidity after the preparation, a gasseparation membrane having high gas treating performance as aimed in theinvention cannot be obtained.

Suitable polar solvents include, but are not limited to, phenol-basedsolvents, such as phenols, e.g., phenol, cresol, and xylenol, catecholshaving two hydroxyl groups on a benzene ring, and halogenated phenols,e.g., 3-chlorophenol, 4-chlorophenol (hereinafter sometimes abbreviatedas PCP), 4-bromophenol, and 2-chloro-5-hydroxytoluene; amide solvents,such as N-methyl-2-pyrrolidone, N,N-dimethylformamide,N,N-dimethylacetamide, and N,N-diethylacetamide; and mixtures thereof.

The manner of the polymerization and imidation reaction in step 2 is notparticularly restricted as long as the reaction results in formation ofa di- or multi-block copolymer having the one block composed ofpolyimide component A and one block composed of polyimide component Bjoined end-to-end. Usually, formation of the di- or multi-blockcopolymer can conveniently be accomplished by conducting polymerizationand imidation until the multi-component polyimide blend solutionincreases its number-averaged molecular weight preferably twice or more,more preferably five times or more. The multi-component polyimide blendsolution resulting from the polymerization and imidation reaction ofstep 2 suitably has a number-averaged polymerization index of 20 to1000, preferably 20 to 500, more preferably 30 to 200. A blend solutionwith too small a number-averaged polymerization index has too low asolution viscosity, which makes film formation in step 3 difficult, andthe resulting asymmetric membrane has reduced mechanical strength. Ablend solution with too large a number-averaged polymerization index isliable to macrophase separation and has too high a solution viscosity,which also makes film formation in step 3 difficult. The solutionviscosity (rotational viscosity) of the multicomponent polyimide blendsolution obtained in step 2 is a characteristic requirement for shapingthe solution into a prescribed form (e.g., a hollow fiber form) and forstabilizing the form as shaped in the formation of an asymmetricmembrane by phase inversion. In the present invention, it is advisableto adjust the solution viscosity of the multicomponent polyimide blendsolution in a range of 20 to 17000 poise, preferably 100 to 15000 poise,more preferably 200 to 10000 poise, at 100° C. A polyimide solutionhaving a solution viscosity falling within the recited range can be, forexample, spun through a spinneret into a desired shape such as a hollowfiber in a stable manner in a spinning procedure in the manufacture ofhollow fiber asymmetric membrane. With a solution viscosity lower than20 poise or higher than 17000 poise, the solution has difficulty instabilizing the shape as extruded, such as a hollow fiber geometry.

A multicomponent polyimide blend solution having an appropriatenumber-averaged polymerization index and solution viscosity can easilybe obtained by (1) preparing a multi-component polyimide blend solutionhaving a total diamine component to total tetracarboxylic acid componentmolar ratio (total number of moles of a diamine component(s)/totalnumber of moles of a tetracarboxylic acid component(s)) ranging from0.95 to 0.99 or from 1.01 to 1.05, more preferably from 0.96 to 0.99 orfrom 1.015 to 1.04 in step 1 and (2) further polymerizing and imidatingthe resulting blend solution in step 2.

It is preferred that the amount of the solvent in the multi-componentpolyimide blend solutions of steps 1 and 2 be adjusted to give a polymerconcentration of 5% to 40% by weight, preferably 8% to 25% by weight,more preferably 9% to 20% by weight. At a polymer concentration lowerthan 5% by weight, the solution is liable to produce an asymmetricmembrane with defects in a phase inversion process, which, when used asa gas separation membrane, has poor gas permeation performance. At apolymer concentration exceeding 40% by weight, the resulting asymmetricmembrane tends to have a reduced rate of gas permeation due to anincreased thickness in its dense layer or a reduced porosity in itsporous layer. As far as the use as a gas separation membrane isconcerned, it is difficult to obtain a satisfactory asymmetric membranefrom such a solution.

Step 3 is characterized in that an asymmetric membrane is formed byphase inversion of the multi-component polyimide blend solution obtainedin step 2. A phase inversion process is a known film formation techniquein which a polymer solution is brought into contact with a coagulationbath to cause phase inversion. In the present invention, what we call adry/wet process is conveniently employed. The dry/wet process, which wasproposed by Loeb, et al. (see, e.g., U.S. Pat. No. 3,133,132), involvesforming a polymer solution into film, evaporating the solvent from thefilm of the polymer solution, which can lead to the development of adense layer, then immersing the film into a coagulating bath (a solventmiscible with the solvent of the polymer solution and incapable ofdissolving the polymer) to induce phase separation thereby to form finepores, which can lead to the formation of a porous layer. In step 3according to the present invention, macrophase-separation is suppressed,and microphase separation is allowed to proceed to form a polyimideasymmetric membrane having a properly controlled composition of thefluorine-containing polyimide in its dense layer.

The asymmetric hollow fiber membrane of the invention can beconveniently produced by adopting a dry/wet spinning process to carryingout step 3. The dry/wet spinning process is application of theaforementioned dry/wet phase inversion process to a polymer solutionhaving been extruded through a spinneret into a hollow fiber geometry tomanufacture an asymmetric hollow fiber membrane. More specifically, apolymer solution is forced through a spinneret into a hollow fibergeometry. Immediately thereafter, the extruded hollow fibers are passedthrough an air or nitrogen gas atmosphere and then immersed in acoagulation bath substantially incapable of dissolving the polymercomponents and compatible with the solvent of the polymer blend solutionto form an asymmetric structure. Subsequently, the hollow fibers aredried and, if desired, heat treated to make a separation membrane.

In order to stably maintain the shape (e.g., hollow fiber) immediatelyafter the extrusion, the multi-component polyimide blend solution to beextruded through the spinneret preferably has a solution viscosity of 20to 17000 poise, more preferably 100 to 15000 poise, even more preferably200 to 10000 poise, at the spinning temperature (e.g., 100° C.) aspreviously stated. Coagulation is preferably carried by first immersionin a first coagulation bath where the membrane is coagulated to anextent enough to retain its shape (e.g., hollow fiber), taking up themembrane by a guide roll, and second immersion in a second coagulatingbath where the membrane is thoroughly coagulated. Drying of thecoagulated membrane is efficiently conducted by replacing thecoagulating liquid with a solvent such as a hydrocarbon prior to drying.The heat treatment, if performed, is preferably at a temperature lowerthan the softening point or the secondary transition point of everypolymer constituting the multi-component polyimide.

The multi-component polyimide blend solution used in step 3 of theinvention is a blend solution of a multi-component polyimide having anappropriate polymerization index and containing at least a polymer fromthe polyimide A component, a polymer from the polyimide B component and,in addition, a di- or multi-block copolymer having the polyimidecomponent A and the polyimide component B bonded end-to-end, as obtainedby the polymerization and imidation reaction in step 2.

While the blend solution undergoes phase separation in the phaseseparation membrane formation, the di- or multi-block copolymer havingthe polyimide component A and the polyimide component B bondedend-to-end functions as a kind of a surfactant between the polymer ofthe polyimide component A and the polymer of the polyimide component B,which are incompatible to each other. To put it differently, the di- ormulti-block copolymer is segregated to the interface between the domainsmade of the polyimide component A and the domains of the polyimidecomponent B to block the repulsive interactions between the differentdomains. In this way, the di- or multi-block copolymer allows desirablemicrophase-separation to proceed while suppressingmacrophase-separation.

Fluorine-containing polyimide is considered to hardly precipitate in thedense layer in the asymmetric membrane formation by phase inversionbecause it is generally more soluble than fluorine-free polyimide.Nonetheless, because fluorine-containing polyimide has a low surfacefree energy, it is thermodynamically concentrated on the membranesurface thereby to reduce the enthalpy of the membrane surface.Fluorine-containing polyimide's existing in the dense layer in a higherfraction is assumed attributed to this thermodynamic effect.

The asymmetric membrane obtained by the invention is a polyimideasymmetric membrane having a dense layer and a porous layer. Itcomprises multi-component polyimide containing a fluorine-containingpolyimide, with the fluorine-containing polyimide existing in the denselayer in a higher composition. That is, the asymmetric membrane containsa fluorine-containing polyimide having relatively good permeationcharacteristics in a high fraction in the dense layer required to havehigh permeation performance as a gas separation membrane, whilecontaining a to polyimide having no or little fluorine content andtherefore exhibiting relatively high mechanical strength in the porouslayer. Therefore, the asymmetric membrane of the invention is extremelywell suited for use as a gas separation membrane. Seeing that thepolyimide compositions vary in the thickness direction, the asymmetricmembrane can be said to have a gradient structure.

The gradient structure can be ascertained through dynamic secondary ionmass spectrometry (hereinafter abbreviated as dSIMS). dSIMS is atechnique for elemental depth profiling, in which an O₂ ⁺ ion beam isused to sputter species from the membrane surface, and the secondaryions sputtered at various depths are mass analyzed. FIG. 5 is theresults of depth profiling (Atomica Dynamic SIMS4000; O₂ ⁺ ionbombardment current: 15 nA/μm²) of fluorine concentration in a uniformfilm from the surface to the inside, the film formed by casting amulti-component polyimide blend solution containing a di- or multi-blockcopolymer and having an appropriate polymerization index that wasprepared by the process of the invention on a glass substrate anddrying. In FIG. 5, the abscissa represents a depth from the samplesurface, the depth being calculated from the time of sputtering and theaverage rate of etching previously determined based on the time requiredfor sputter etching a deuterated polystyrene cover layer provided on thesurface of the sample with a known thickness. In the plots of thefluorine concentrations at a depth 0 nm (surface) to 150 nm, a regionwith a high fluorine concentration is observed from the surface up to adepth of about 50 nm.

The same analysis was conducted on a uniform film obtained, in contrast,by casting a polyimide dope solution prepared by a conventionalpolymerization process on a glass substrate and drying. The results areplotted in FIG. 6. There is observed no noticeable gradient structure inthe fluorine distribution near the surface.

The polyimide asymmetric membrane according to the present inventionpreferably has a ratio of the fluorine atom concentration (Φ_(s)) of thedense layer's surface, measured by X-ray photoelectron spectroscopy, tothe average fluorine atom concentration (f) of the multi-componentpolyimide forming the membrane, Φ_(s)/f, from 1.1 to 1.8.

When raw materials of the same composition as used to prepare themulti-component polyimide containing a fluorine-containing polyimide arepolymerized and imidated in a usual manner to prepare a randomcopolyimide solution, which is then formed into a polyimide asymmetricmembrane by the dry/wet processing, the resulting membrane has afluorine atom concentration ratio Φ_(s)/f of about 1.0. In contrast, thepolyimide asymmetric membrane of the invention has thefluorine-containing polyimide more distributed in the dense layer andpreferably has a fluorine atom concentration ratio Φ_(s)/f of from 1.1to 1.8, more preferably of from 1.2 to 1.7.

The polyimide asymmetric membrane used as a gas separation membrane hasa dense layer and a porous layer. The dense layer has such denseness asto have substantially different permeation rates depending on gasspecies (for example, the helium to nitrogen gas permeation rate ratiois 1.2 or more at 50° C.) and therefore functions to separate gasspecies. On the other hand, the porous layer has such porosity as tohave practically no gas separation functionality. The pore size is notnecessarily uniform. The porous layer may have the pore size decreasingfrom its surface to the inside and may continuously lead to a denselayer. The polyimide asymmetric membrane of the invention has no defectsin its dense layer and exhibits high gas separation performance. Theasymmetric membrane is not limited in form, thickness, dimension, etc.and may be, for example, a flat film or a hollow fiber. For use as a gasseparation membrane, a suitable thickness of the dense layer is about 1to 1000 nm, preferably about 20 to 200 nm, and that of the porous layeris about 10 to 2000 μm, preferably about 10 to 500 μm. For use as ahollow fiber gas separation membrane, in particular, a suitable innerdiameter is about 10 to 3000 μm, preferably about 20 to 900 μm, and asuitable outer diameter is about 30 to 7000 μm, preferably about 50 to1200 μm. A hollow fiber membrane is preferably an asymmetric hollowfiber membrane with the dense layer outside.

It is preferred for the polyimide asymmetric membrane obtained by theinvention to have high gas separation performance and mechanicalstrength for practical use. Specifically, the hydrogen permeation rate(P′_(H2)) is preferably 4.0×10⁻⁴ cm³(STP)/cm²·sec·cmHg or more, morepreferably 5.0×10⁻⁴ cm³(STP)/cm²·sec·cmHg or more. The ratio of thehydrogen gas permeation rate (P′_(H2)) to nitrogen gas permeation rate(P′_(N2)), P′_(H2)/P′_(N2), is preferably 20 or greater, more preferably45 or greater. The tensile elongation at break is 15% or more. Inparticular, a hollow fiber membrane has a tensile elongation at break of15% or more. The helium gas permeation rate (P′_(He)) is preferably4.0×10⁻⁴ cm³(STP)/cm²·sec·cmHg or more, more preferably 5.0×10⁻⁴cm³(STP)/cm²·sec·cmHg or more. The ratio of the helium gas permeationrate (P′_(He)) to nitrogen gas permeation rate (P′_(N2)),P′_(He)/P′_(N2), is preferably 20 or greater, more preferably 45 orgreater. The tensile elongation at break is 15% or more. In particular,a hollow fiber membrane has a tensile elongation at break of 15% ormore.

Where a hollow fiber has a tensile elongation at break of less than 15%,the hollow fiber membrane will easily cut or break when incorporatedinto a module. Due to a failure to fabricate a module on an industrialscale, such hollow fibers are impractical. Hollow fibers having atensile elongation at break of 15% or more are easy to assemble into amodule and therefore practical. Hollow fibers with a tensile elongationat break of less than 15% are also impractical in that they tend to cutwhile used (particularly with high pressure gas introduced) and hencemust be used under limited conditions.

Accordingly, the asymmetric hollow fiber gas separation membrane of thepresent invention can conveniently be fabricated into a module in aconventional manner. An ordinary gas separation membrane module isformed as follows. About 100 to 200,000 hollow fiber membranes ofappropriate length are bound into a bundle. The hollow fibers of thebundle are secured at both ends thereof with a tube sheet made of athermoplastic resin, etc. to make a hollow fiber membrane element withboth ends of the individual fibers open. The resulting hollow fibermembrane element is set in a container having at least a mixed gasinlet, a permeate outlet, and a retentate (non-permeate) outlet in sucha manner that the space leading to the inside of the individual hollowfibers and the space leading to the outside of the hollow fibers areisolated from each other. A gas mixture is fed from the mixed gas inletto the space in contact with the inside or outside of the hollow fibermembrane. While the mixed gas flows along the hollow fiber membrane, aspecific component in the mixed gas selectively passes through themembrane. The permeate gas is discharged from the permeate outlet, whilethe retentate gas that has not passed through the membrane is dischargedfrom the retentate outlet, thus accomplishing gas separation.

Seeing that the gas separation membrane comprising the polyimideasymmetric membrane of the invention is a practical high-performance gasseparation membrane having excellent gas permeation characteristics andexcellent mechanical strength as described above, it is useful forseparation between hydrogen gas and a hydrocarbon gas such as methane,separation between hydrogen and nitrogen, separation between helium andnitrogen, separation between carbonic acid gas and a hydrocarbon gassuch as methane, and separation between oxygen and nitrogen, and soforth. Inter alia, the gas separation membrane of the invention is wellsuited for separation between hydrogen gas and a hydrocarbon gas such asmethane, separation between hydrogen gas and nitrogen gas, andseparation between oxygen gas and nitrogen gas.

A polymerization index used in the present invention can be determinedby previously examining the correlation between number-averagedpolymerization index and solution viscosity by measuring an imidationratio through gel-permeation chromatography (GPC) or infraredspectroscopy and measuring the solution viscosity of a reactionsolution. GPC measurement was adopted for reaction solutions having animidation ratio of 90% or higher, and infrared spectroscopy was used forthose having an imidation ratio less than 90%.

GPC measurement was carried out as follows using an HPLC system of800-series available from JASCO Corp. equipped with a Shodex KD-806Mcolumn (column temperature: 40° C.) and Intelligent UV/Vis detector(absorption wavelength: 350 nm) for unknown samples or a differentialrefractive index detector for standards (polyethylene glycol).N-Methyl-2-pyrrolidone containing 0.05 mol/l each of lithium chlorideand phosphoric acid was used as a solvent. The flow rate of the solventwas 0.5 ml/min, and the sample concentration was about 0.1%. Dataacquisition and processing are done by JASCO-JMBS/Bowrin software. Datawas acquired twice a second to prepare a chromatogram of a sample.Polyethylene glycols having a molecular weight of 82, 250, 28, 700, 6,450, 1, and 900 were used as standards to provide chromatograms, fromwhich peaks were detected to prepare a calibration curve showing therelation between retention time and molecular weight. The molecularweight analysis of an unknown sample was performed as follows. Molecularweight M_(i) at each retention time was obtained from the calibrationcurve, and the fraction of the height h_(i) of the chromatogram at eachretention time to the total height (W_(i)=h_(i)/Σh_(i)) was obtained.The number-averaged molecular weight Mn and weight-averaged molecularweight Mw of the sample are calculated from 1/[Σ(W_(i)/M_(i))] andΣ(W_(i)·M_(i)), respectively.

The number-averaged polymerization index N was obtained by dividing thenumber-averaged molecular weight Mn by a monomer unit molecular weight<m> averaged according to the composition of the monomer components usedto commence polymerization.N=Mn/<m>

The monomer unit molecular weight <m> was obtained as follows. When aplurality of tetracarboxylic acid components (molecular weight: m_(1,i);molar ratio: R_(1,i); ΣR_(1,i)=1; i=1, 2, 3, . . . , n₁) and a pluralityof diamine components ((molecular weight: m_(2,j); molar ratio: R_(2,j);ΣR_(2,j)=1; j=1, 2, 3, . . . , n₂) are charged, the monomer unitmolecular weight <m> was calculated according to equation below.<m>=(ΣR _(1,i) m _(1,i) +ΣR _(2,j) m _(2,j))−36

Imidation ratio measurement by infrared spectroscopy was performed byattenuated total reflection-Fourier transform infrared spectrometry(ATR-FTIR) on Spectrum-One FTIR spectrophotometer (Perkin Elmer). Theabsorbance A of C—N stretching vibration (wave number: about 1360 cm⁻¹)of an imide bond was standardized taking the absorbance A_(I) ofaromatic ring C═C in-plane vibration (wave number: about 1500 cm⁻¹) asan internal standard. The same sample was analyzed in the same mannerafter heat treatment at 190° C. for 5 hours, and the absorbance A_(S) ofC—N stretching vibration was standardized using the absorbance A_(SI) ofaromatic ring C═C in-plane vibration as an internal standard. Theimidation ratio p_(I) is calculated by dividing the former standardizedvalue (A/A_(I)) by the latter standardized value (A_(S)/A_(SI)).p _(I)=(A/A _(I))/(A _(S) /A _(SI))

The line connecting valleys on both sides of an absorption band wastaken as a base line from which the peak intensity of absorbance wasmeasured.

The number-averaged polymerization index N is calculated from theimidation ratio according to equation below.N=(1+r)/(2r(1−p _(I))+(1−r))

In the equation above, r is a compositional ratio of the total number ofmoles of a tetracarboxylic acid component(s) to the total number ofmoles of a diamine component(s) in a polyimide. Where a diaminecomponent is more than a tetracarboxylic acid component, the reciprocalof the ratio is used as r. In either case, r is not greater than one.p_(I) is an imidation ratio.

In the present invention, the composition of a fluorine-containingpolyimide in the dense layer can be found by determining the fluorineatom concentration Φ_(S) in the dense layer's surface by X-rayphotoelectron spectroscopy (XPS or ESCA).

An atomic concentration Φ_(j) of a specific element j is represented byequation:Φ_(j) =N _(j) /ΣN _(i)where N_(i) is the number of atoms of a detectable element contained ina polyimide (hydrogen and helium are undetectable); N_(j) is the numberof atoms of the specific element j (the subscript represents an elementspecies); and ΣN_(i) is the sum of the numbers of atoms of all thedetectable elements in the polyimide.

XPS is carried out by irradiating the dense layer's surface of apolyimide asymmetric membrane with X-rays to emit electrons (calledphotoelectrons) from each orbital of each element contained in thepolyimide into the vacuum, and measuring the intensity of the emittedphotoelectrons versus the kinetic energy. In order to minimize damage tothe polyimide surface, it is desirable to use monochromatized AlKα raysfree from X-ray components unnecessary for XPS.

Binding energy E_(b), of electrons in atoms in a substance is calculatedfrom the photoelectron kinetic energy E_(k) according to equation:E _(b) =hν−E _(k) −Wwhere hν is an incident energy of X-rays; and W is a work function of aspectrometer used to detect photoelectrons.

Since the binding energy depends almost entirely on the atom species andthe electron orbital, detection of all elements should be possibletheoretically only if the incident energy of X-rays is correctly chosen.In fact, however, hydrogen and helium cannot be detected because of thesmall probability of the electron on each orbital being excited byX-rays (photoionization cross section).

The intensity I_(j) of the photoelectron emitted by X-ray bombardmentfrom orbital l of specific element j present in polyimide is representedby equation:I_(j)=N_(j)σ_(j) ^(l)λ_(j) ^(l)A_(j) ^(l)Rwhere N_(j) is the number of atoms of element j per unit volume; σ_(j)^(l) is the l-shell photoionization cross section of element j; λ_(j)^(l) is the mean-free-path for inelastic scattering of an electronemitted from l-shell of element j traveling in polyimide; A_(j) ^(l) isa function of an instrument on the electron emitted from l-shell ofelement j; and R is the surface roughness coefficient of a polyimideasymmetric membrane.

The photoionization cross section σ_(i) ^(l) and the mean-free-path forinelastic scattering λ_(j) ^(l) are known values. A_(j) ^(l) is a valuedecided from the instrument and measurement conditions. The value Rvaries from sample to sample but are cancelled by taking an intensityratio and is therefore unnecessary in the calculations hereinafterdescribed to obtain an atomic concentration.

In the present invention, the atomic concentration Φ_(j) of specificelement j in polyimide was obtained using the measured photoelectronintensity I_(j) in accordance with equation:Φ_(j)=(I _(j) /S _(j))/Σ(I _(i) /S _(i))where S_(j)=σ_(j) ^(l)λ_(j) ^(l)A_(j) ^(l); S_(i) represents relativesensitivity for element i; and Σ(I_(i)/S_(i)) represents the sum ofphotoelectron intensities of all the detectable elements i present inpolyimide divided by the respective relative sensitivities.

The relative sensitivity S_(j) can be decided separately using astandard substance whose atomic concentration is known. While relativesensitivity S′_(j) that is furnished by an XPS system manufacturer maybe utilized for the sake of convenience, a relative sensitivity wasdecided in the present invention by using a polyimide having a singlecomposition (i.e., a homopolyimide obtained from one tetracarboxylicacid component and one diamine component) the atomic concentration ofwhich is known.

When a sample made of a polyimide having a single composition (i.e., ahomopolyimide obtained from one tetracarboxylic acid component and onediamine component) is analyzed, it is expected that the surface atomicconcentration Φ_(s,j) and the average atomic concentration f_(j) are insubstantial agreement. If a relative sensitivity S′_(j) supplied by anXPS system manufacturer, etc., i.e., a relative sensitivity factorcorrected by the instrumental function, is used as such in themeasurement of surface atomic concentration Φ_(s, j), there is oftendisagreement between Φ_(s,j) and f_(j). The disagreement is attributedto the fact that the relative sensitivity S′_(j) is a valueexperimentally obtained using a standard substance other than polyimide.Therefore, the value S′_(j) was corrected so that the surface atomicconcentration Φ_(s,j) and the average atomic concentration f_(j) of asample made of a homopolyimide having a single composition may agreewith each other, and the thus corrected value was used as relativesensitivity S_(j) in the determination of a surface atomic concentrationof a polyimide material. Namely, the relative sensitivity S_(j) as usedin the invention is represented by equation:S _(j) =S′ _(j)×α_(j)where α_(j) is a correction factor for making a relative sensitivityS′_(j) determined for element j by using other standard material thanpolyimide applicable to a polyimide material.

In the present invention, the correction factor was obtained for everyelement through measurements, and a relative sensitivity S_(j) ascorrected by the correction factor was used.

In the invention, the photoelectron intensity I_(j) is obtained from thephotoelectron peak area under the photoelectron spectrum measured byXPS. Of photoelectron peaks a peak for a transition with a relativelylarge photoionization cross section is preferably made use of. Usually,a photoelectron peak for a transition having a photoionization crosssection 10% larger than that of the C 1s orbital is conveniently used.In the invention, a photoelectron peak from the 1s orbital is preferablymade use of for fluorine. A photoelectron peak from the 1s orbital waspreferably made use of for carbon; the 1s orbital for nitrogen; the 1sorbital for oxygen; and the 2p orbital for sulfur.

The photoelectron spectrum has a background due to inelastic scatteringof photoelectrons when emitted from a sample into the vacuum. Thebackground is subtracted from each photoelectron peak used to determinean atomic concentration to give a residual area as I_(j).

When the asymmetric membrane to be XPS analyzed is a hollow fibermembrane, an X-ray probe size should be less than the hollow fiberdiameter. The hollow fiber diameter being 30 μm or larger, generallyabout 100 μm or larger, a probe size of about 100 μm or smaller issuitably used. A probe size of about 20 μm is preferably used.

Because the polyimide surface is charged as photoelectrons are ejected,it is preferred to neutralize the surface charges by, for example, anelectron beam.

In XPS measurement, the depth of measurement from the specimen surfacevaries depending on the emission take-off angle θ measured relative tothe specimen surface. Ninety-five percent of the photoelectrons detectedby XPS are those emitted from a depth up to 3λ_(j) ¹ sin θ. The range ofθ is not particularly limited as long as measurement is possible. Forexample, an angle of 45° is conveniently used. The depth of analysis isup to several nanometers from the specimen surface. Therefore, theatomic concentration as measured by XPS is surface atomic concentrationΦ_(s,j) within a thickness ranging from the surface to a depth ofseveral nanometers.

On the other hand, the average atomic concentration f_(j) for element jcontained in multi-component polyimide forming the whole membrane isrepresented by equation:f _(j) =Σm _(k) n _(k) /Σm _(k) N _(k)where n_(k) is the number of atoms of element j contained in monomer k(when monomer k is a tetracarboxylic acid or an anhydride thereof, andelement j is oxygen, n_(k) is the number of oxygen atoms except theoxygen atoms released in the form of condensation water at the time ofpolymerization into polyimide); N_(k) is the total number of all theXPS-detectable atoms in monomer k (when monomer k is a tetracarboxylicacid or an anhydride thereof, N_(k) is the number of all the detectableatoms except the oxygen atoms released in the form of condensation wateron polymerization to polyimide); m_(k) is the molar fraction of monomerk in multi-component polyimide forming the membrane; and Σ meanssummation of data for all the monomers k in the multi-componentpolyimide.

The fluorine atom concentration (f) of a membrane as a whole in thepresent invention is a value calculated according to the above-describedequation.

A process of producing an asymmetric membrane of multi-componentpolyimide according to the invention and the characteristics of theresulting asymmetric membrane are then described hereunder. It should benoted that the invention is not limited to Examples hereinafter given.

Methods of measurements carried out in the invention are explainedbelow.

(1) Preparation of Polyimide Film

A polyimide solution prepared with its solution viscosity adjusted to 50to 1000 poise at 100° C. was filtered through a 400 mesh net and allowedto stand at 100° C. for deaeration. The resulting polyimide solution at50° C. was cast on a glass plate using a doctor knife set at 0.5 mm or0.2 mm clearance and heated in an oven at 100° C. for 3 hours toevaporate the solvent, and further heat treated in an oven at 300° C.for 1 hour to obtain a polyimide film as a sample for helium gaspermeability coefficient measurement.

(2) Measurement of Helium Gas Permeability Coefficient (P_(He)),Nitrogen Gas Permeability Coefficient (P_(N2)), and Helium to NitrogenPermeability Coefficient Ratio (P_(He)/P_(N2))

Helium gas permeability coefficient measurement was carried out by ahigh-vacuum time-lag method. The polyimide film was mounted in apermeation cell. The cell was kept at 80° C. and evacuated to a highvacuum of 10⁻⁵ Ton with a vacuum pump. Helium gas was fed to the feedside of the film at a pressure of 2.5 kgf/cm², and an increase inpressure in the permeate side of the film was recorded with time. Thehelium gas permeability coefficient (P_(He)) was calculated from thethickness and effective area of the film, the volume of the permeateside, the pressure in the feed side, etc. The nitrogen gas permeabilitycoefficient (P_(N2)) of the film was determined in the same manner usingthe same polyimide film, except for replacing helium gas with nitrogengas. A helium to nitrogen permeability coefficient ratio (P_(He)/P_(N2))was calculated from the thus obtained helium gas permeabilitycoefficient (P_(He)) and nitrogen gas permeability coefficient (P_(N2)).

(3) Measurement of Helium Gas and Nitrogen Gas Permeation Performance ofHollow Fiber Membrane

An element for permeation performance evaluation having an effectivelength of 10 cm was prepared using 15 hollow fiber membranes, astainless pipe (container), and an epoxy resin adhesive. The element wasput into the stainless steel container to make a pencil module. Heliumgas was fed to the module under a given pressure to measure the permeateflow rate. The helium gas permeation rate was calculated from the amountof permeate helium gas, the feed pressure, and the effective membranearea. A nitrogen gas permeation rate was measured in the same manner.These measurements were taken at 80° C.

(4) Measurement of Tensile Strength and Elongation at Break of HollowFiber Membrane

Measurements were made with a tensile tester on an effective length of20 mm at a pulling speed of 10 mm/min at a measuring temperature of 23°C. The cross-sectional area of a hollow fiber was obtained by observinga cross-section of a hollow fiber under an optical microscope andmeasuring the size.

(5) Measurement of Rotational Viscosity

The solution viscosity of a polyimide solution was measured with arotational viscometer (shear rate of rotor: 1.75 sec⁻¹) at 100° C.

(6) Measurement of Fluorine Atom Concentration on Dense Layer Surface byX-Ray Photoelectron Spectroscopy

X-Ray photoelectron spectroscopy was carried out with Quantum 2000Scanning ESCA Microprobe from PHI. Monochromatized AlKα rays were usedas X-rays. The X-ray probe beam diameter was 20 μm. The take-off anglewas 45°. A flood electron gun was used to neutralize the charges on thesample surface. The area of each of the peaks from the C 1s orbital, theN 1s orbital, the O 1s orbital, the F 1s orbital, and the S 2sp orbitalwas obtained after removing the background. Multipack software version6.1A (1999) from PHI was used for processing the photoelectron peaks andcalculating the atomic concentrations. A relative sensitivity S′_(j)obtained by correcting the corresponding relative sensitivity factorASF_(j) supplied by PHI for each photoelectron peak by the instrumentalfunction of the apparatus (the permeation function of the spectroscope)is shown in Table 1 below. In Table, the S values are relativelyexpressed taking the value for F 1s as 1.

TABLE 1 Photoelectron Peak C 1s N 1s O 1s F s S 2p Relative Sensitivity0.31 0.49 0.73 1 0.80 (S′)

A homopolyimide film consisting of 6FDA and TSN and a homopolyimide filmconsisting of 6FDA and DABA were prepared, and the atomic concentrationfor every element of the polyimide was determined using S_(j) obtainedby correcting the relative sensitivity S′_(j)(S_(j)=S′_(j)×α_(j)). Thecorrection factor (α) used for each element is shown in Table 2 below.

TABLE 2 Photoelectron Peak C 1s N 1s O 1s F 1s S 2p Relative Sensitivity1.08 1.01 1.02 0.93 0.96 Correction Factor (α)

Reference Example 1

In a separable flask were put 12.44 g of2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), 4.92 gof dimethyl-3,7-diaminodibenzothiophene 5,5-dioxide (TSN), 1.64 g of3,5-diaminobenzoic acid (DABA), and 102 g of p-chlorophenol (PCP) as asolvent, and the system was allowed to polymerize and imidate at 190° C.for 31 hours to obtain a polyimide solution having a polymerconcentration of 15 wt % and a rotational viscosity of 446 poise.

A polyimide film formed of the resulting polyimide solution had a heliumgas permeability coefficient (P_(He)) of 1.1×10⁻⁸cm³(STP)·cm/cm²·sec·cmHg and a helium to nitrogen gas permeabilitycoefficient ratio (P_(He)/P_(N2)) of 37.

Reference Example 2

In a separable flask were put 1236 g of3,3′,4,4′-biphenyltetracarboxylic acid dianhydride (s-BPDA), 1135 g ofTSN, and 165 g of PCP as a solvent, and the system was allowed topolymerize and imidate at 190° C. for 25 hours to obtain a polyimidesolution having a polymer concentration of 11.8 wt % and a rotationalviscosity of 600 poise.

A polyimide film formed of the resulting polyimide solution had a heliumgas permeability coefficient (P_(He)) of 2.2×10⁻⁹cm³(STP)·cm/cm²·sec·cmHg and a helium to nitrogen gas permeabilitycoefficient ratio (P_(He)/P_(N2)) of 110. The polyimide film had a totensile strength at break of 260 MPa, a Young's modulus of 5925 MPa, andan elongation at break of 24%.

(7) Preparation of Asymmetric Hollow Fiber Membrane

The asymmetric hollow fiber membranes used in Examples were prepared bydry/wet spinning method. A polyimide solution or a polyimide blendsolution was filtered through a 400 mesh net and spun through a hollowfiber spinneret (outer diameter of annular opening: 1000 μm; slit widthof annular opening: 200 μm; outer diameter of core opening: 400 μm) at71° C. The spun hollow fibers were passed through a nitrogen atmosphereand immersed in a coagulation bath of a 75 wt % ethanol aqueous solutionat 0° C. The wet fibers were then immersed in ethanol at 50° C. for 2hours to remove the solvent. The fibers were further immersed inisooctane at 70° C. for 3 hours for solvent replacement, dried at 100°C. for 30 minutes in an absolute dry condition, and heat treated at 300°to 320° C. for 1 hour. The fibers were further treated with silicone oilto improve the surface slip. The resulting individual hollow fibers hadan outer diameter of around 400 μm, an inner diameter of around 200 μm,and a thickness of about 100

Example 1

In a separable flask, 12.36 g of s-BPDA and 11.35 g of TSN (0.985 partsby mole of a diamine per part by mole of an acid dianhydride; B/A=0.985)were polymerized and imidated in 165 g of PCP as a solvent at 190° C.for 22 hours to obtain a polyimide B solution having a polymerconcentration of 11.8 wt %. The polyimide B was found to have anumber-averaged polymerization index N_(B) of 74 as measured by theabove-described GPC method. To the polyimide solution were added 12.44 gof 6FDA, 5.21 g of TSN, 1.73 g of DABA (1.085 parts by mole of diaminesper part by mole of an acid dianhydride), and 20 g of PCP as a solvent.The resulting multi-component polyimide blend solution was subjected tofurther polymerization and imidation at 190° C. for 8 hours to obtain amulticomponent polyimide blend solution having a rotational viscosity of2046 poise and a polymer concentration of 18 wt %. The number-averagedpolymerization index of the multicomponent polyimide was found to be 41as measured by the above-described GPC method.

An asymmetric membrane was prepared using the resulting multicomponentpolyimide blend solution. The characteristics of the asymmetric membranewere measured. The results obtained are shown in Table 3 below.

Example 2

In a separable flask, 12.36 g of s-BPDA and 11.35 g of TSN werepolymerized and imidated in 169 g of PCP as a solvent at 190° C. for 27hours to obtain a polyimide B solution having a polymer concentration of11.6 wt %. The polyimide B was found to have a number-averagedpolymerization index N_(B) of 75 as measured by the above-described GPCmethod. To the polyimide solution were added 12.44 g of 6FDA, 4.17 g ofTSN, 3.77 g of 3,3′-diaminodiphenyl sulfone (hereinafter sometimesabbreviated as MASN), and 20 g of PCP as a solvent. The resultingmulticomponent polyimide blend solution was subjected to furtherpolymerization and imidation at 190° C. for 8 hours to obtain amulticomponent polyimide blend solution having a rotational viscosity of1693 poise and a polymer concentration of 18 wt %. The number-averagedpolymerization index of the multicomponent polyimide was found to be 41as measured by the above-described GPC method.

An asymmetric membrane was prepared using the multi-component polyimideblend solution. The characteristics of the resulting asymmetric membranewere measured. The results obtained are shown in Table 3 below.

Comparative Example 1

In a separable flask, 12.71 g of s-BPDA, 12.79 g of 6FDA, 16.20 g ofTSN, and 3.67 g of MASN (1.025 parts by mole of diamines per part bymole of acid dianhydrides) were polymerized and imidated in 196 g of PCPas a solvent at 190° C. for 54 hours to obtain a polyimide solutionhaving a rotational viscosity of 1097 poise and a polymer concentrationof 18 wt %. The polyimide was found to have a number-averagedpolymerization index of 45 as measured by the above-described GPCmethod.

An asymmetric membrane was prepared using the multi-component polyimideblend solution. The characteristics of the resulting asymmetric membranewere measured. The results obtained are shown in Table 3 below.

The composition of the raw materials used in Comparative Example 1 isnearly the same as that of Example 2, but the resulting film had Φ_(s)/fof 1.02 and a tensile elongation at break as low as 7%.

Example 3

In a separable flask, 6.36 g of s-BPDA, 12.79 g of 6FDA, 8.10 g of TSN,3.67 g of MASN, and 1.12 g of DABA were polymerized and imidated in 171g of PCP as a solvent at 190° C. for 27 hours to obtain a polyimide Asolution having a polymer concentration of 15.0 wt %. The polyimide Awas found to have a number-averaged polymerization index N_(A) of 31 asmeasured by the above-described GPC method. To the polyimide solutionwere added 6.36 g of s-BPDA, 6.07 g of TSN, and 20 g of PCP as asolvent. The resulting multicomponent polyimide blend solution wassubjected to further polymerization and imidation at 190° C. for 19hours to obtain a multicomponent polyimide blend solution having arotational viscosity of 1246 poise and a polymer concentration of 18 wt%. The number-averaged polymerization index of the multi-componentpolyimide was found to be 46 as a result of the above-described GPCmeasurement.

An asymmetric membrane was prepared using the multi-component polyimideblend solution. The characteristics of the resulting asymmetric membranewere measured. The results obtained are shown in Table 3 below.

Example 4

In a separable flask, 12.36 g of s-BPDA and 11.35 g of TSN werepolymerized and imidated in 165 g of PCP as a solvent at 190° C. for 27hours to obtain a polyimide B solution having a polymer concentration of11.8 wt %. The polyimide B was found to have a number-averagedpolymerization index N_(B) of 76 as measured by the above-described GPCmethod. To the polyimide solution were added 12.44 g of 6FDA, 2.08 g ofTSN, 3.77 g of MASN, 1.16 g of DABA, and 20 g of PCP as a solvent. Theresulting multi-component polyimide blend solution was subjected tofurther polymerization and imidation at 190° C. for 30 hours to obtain amulticomponent polyimide blend solution having a rotational viscosity of911 poise and a polymer concentration of 18 wt %. The number-averagedpolymerization index of the multicomponent polyimide was 45 as measuredby the above-described GPC method.

An asymmetric membrane was prepared using the multicomponent polyimideblend solution. The characteristics of the resulting asymmetric membranewere measured. The results obtained are shown in Table 3 below.

Example 5

In a separable flask, 12.7 g of s-BPDA and 12.15 g of TSN werepolymerized and imidated in 171 g of PCP as a solvent at 190° C. for 27hours to obtain a polyimide B solution having a polymer concentration of12.0 wt %. The polyimide B was found to have a number-averagedpolymerization index N_(B) of 79 as measured by the above-described GPCmethod. To the polyimide solution were added 12.79 g of 6FDA, 2.02 g ofTSN, 3.67 g of MASN, 1.12 g of DABA, and 20 g of PCP as a solvent. Theresulting multi-component polyimide blend solution was subjected tofurther polymerization and imidation at 190° C. for 10 hours to obtain amulticomponent polyimide blend solution having a rotational viscosity of1767 poise and a polymer concentration of 18 wt %. The number-averagedpolymerization index of the multicomponent polyimide was found to be 73as measured by the above-described GPC method.

An asymmetric membrane was prepared using the multi-component polyimideblend solution. The characteristics of the resulting asymmetric membranewere measured. The results obtained are shown in Table 3 below.

Example 6

In a separable flask, 6.36 g of s-BPDA and 6.07 g of TSN werepolymerized and imidated in 171 g of PCP as a solvent at 190° C. for 27hours to obtain a polyimide B solution having a polymer concentration of6.4 wt %. The polyimide B was found to have a number-averagedpolymerization index N_(B) of 57 as measured by the above-described GPCmethod. To the polyimide solution were added 6.36 g of s-BPDA, 12.79 gof 6FDA, 8.10 g of TSN, 3.67 g of MASN, 1.12 g of DABA, and 20 g of PCPas a solvent. The resulting multi-component polyimide blend solution wassubjected to further polymerization and imidation at 190° C. for 19hours to obtain a multi-component polyimide blend solution having arotational viscosity of 1507 poise and a polymer concentration of 18 wt%. The number-averaged polymerization index of the multi-componentpolyimide was found to be 50 as a result of the above-described GPCmeasurement.

An asymmetric membrane was prepared using the multi-component polyimideblend solution. The characteristics of the resulting asymmetric membranewere measured. The results obtained are shown in Table 3 below.

Comparative Example 2

In a separable flask, 12.71 g of s-BPDA, 12.79 g of 6FDA, 14.17 g ofTSN, and 3.67 g of MASN, and 1.12 g of DABA were polymerized andimidated in 191 g of PCP as a solvent at 190° C. for 73 hours to obtaina polyimide solution having a rotational viscosity of 1190 poise and apolymer concentration of 18 wt %. The polyimide was found to have anumber-averaged polymerization index of 49 as measured by theabove-described GPC method.

An asymmetric membrane was prepared using the multi-component polyimideblend solution. The characteristics of the resulting asymmetric membranewere measured. The results obtained are shown in Table 3 below.

The composition of the raw materials used in Comparative Example 2 isnearly the same as that of Example 6, but the resulting film had Φ_(s)/fof 1.04 and a tensile elongation at break as low as 7%.

Example 7

In a separable flask, 12.44 g of 6FDA and 4.37 g of DABA werepolymerized and imidated in 155 g of PCP as a solvent at 120° C. for 2hours to obtain a polyimide A solution having a polymer concentration of9.8 wt %. In order to determine the number-averaged polymerization indexof the reaction solution, a portion of the reaction solution was cast ona slide glass and immersed in ethanol to coagulate. After PCP wasthoroughly removed by solvent displacement, the film was dried in vacuoat room temperature for 5 hours to prepare a specimen for FT-IRanalysis. The imidation rate was determined by the aforementioned methodand found to be 0.63, from which the number-averaged polymerizationindex N_(A) was calculated to be 2.7. To the polyimide solution wereadded 12.36 g of s-BPDA, 11.81 g of TSN, and 20 g of PCP as a solvent.The resulting multi-component polyimide blend solution was subjected tofurther polymerization and imidation at 190° C. for 30 hours to obtain amulti-component polyimide blend solution having a rotational viscosityof 1265 poise and a polymer concentration of 18 wt %. Thenumber-averaged polymerization index of the multi-component polyimidewas found to be 72 as a result of the above-described GPC measurement.

An asymmetric membrane was prepared using the multi-component polyimideblend solution. The characteristics of the resulting asymmetric membranewere measured. The results obtained are shown in Table 3 below.

Example 8

In a separable flask, 12.44 g of 6FDA and 4.37 g of DABA werepolymerized and imidated in 155 g of PCP as a solvent at 120° C. for 1hour to obtain a polyimide A solution having a polymer concentration of9.8 wt %. In order to determine the number-averaged polymerization indexof the reaction solution, a portion of the reaction solution was cast ona slide glass and immersed in ethanol to coagulate. After PCP wasthoroughly removed by solvent displacement, the film was dried in vacuoat room temperature for 5 hours to prepare a specimen for FT-IRanalysis. The imidation rate was determined by the aforementioned methodand found to be 0.52, from which the number-averaged polymerizationindex N_(A) was calculated and found to be 2.1.

To the polyimide solution were added 12.36 g of s-BPDA, 11.81 g of TSN,and 20 g of PCP as a solvent. The resulting multi-component polyimideblend solution was subjected to further polymerization and imidation at190° C. for 30 hours to obtain a multi-component polyimide blendsolution having a rotational viscosity of 1469 poise and a polymerconcentration of 18 wt %. The number-averaged polymerization index ofthe multi-component polyimide was 78 as measured by the above-describedGPC method.

An asymmetric membrane was prepared using the multi-component polyimideblend solution. The characteristics of the resulting asymmetric membranewere measured. The results obtained are shown in Table 3 below.

Example 9

In a separable flask, 23.10 g of 6FDA, 3.66 g of TSN, 6.62 g of MASN,and 2.03 g of DABA were polymerized and imidated in 153 g of PCP as asolvent at 190° C. for 6 hours to obtain a polyimide A solution having apolymer concentration of 18 wt %. The polyimide A had a number-averagedpolymerization index N_(A) of 4.9 as measured by the above-described GPCmethod.

In a separable flask, 21.18 g of s-BPDA and 20.25 g of TSN werepolymerized and imidated in 177 g of PCP as a solvent at 190° C. for 6hours to obtain a polyimide B solution having a polymer concentration of18 wt %. The polyimide B was found to have a number-averagedpolymerization index N_(B) of 51 as measured by the above-described GPCmethod.

Eighty-eight grams of the polyimide A solution and 110 g of thepolyimide B solution were weighed out and mixed in a separable flask.The resulting multi-component polyimide blend solution was subjected tofurther polymerization and imidation at 190° C. for 13 hours to obtain amulti-component polyimide blend solution having a rotational viscosityof 2232 poise and a polymer concentration of 18 wt %. Thenumber-averaged polymerization index of the multi-component polyimidewas found to be 62 by the above-described GPC method.

An asymmetric membrane was prepared using the multi-component polyimideblend solution. The characteristics of the resulting asymmetric membranewere measured. The results obtained are shown in Table 3 below.

Example 10

In a separable flask, 21.18 g of s-BPDA and 20.25 g of TSN werepolymerized and imidated in 177 g of PCP as a solvent at 190° C. for 0.5hours to obtain a polyimide B solution having a polymer concentration of18 wt %. The polyimide B was found to have a number-averagedpolymerization index N_(B) of 6.0 as measured by the above-described GPCmethod.

A hundred and ten grams of the polyimide B solution and 88 g of thepolyimide A solution having a number-averaged polymerization index of4.9 which was obtained in Example 9 were weighed out and mixed in aseparable flask. The resulting multi-component polyimide blend solutionwas subjected to further polymerization and imidation at 190° C. for 19hours to obtain a multi-component polyimide blend solution having arotational viscosity of 1376 poise and a polymer concentration of 18 wt%. The number-averaged polymerization index of the multicomponentpolyimide was found to be 57 by the above-described GPC method.

An asymmetric membrane was prepared using the multi-component polyimideblend solution. The characteristics of the resulting asymmetric membranewere measured. The results obtained are shown in Table 3 below.

Example 11

In a separable flask, 23.10 g of 6FDA, 3.66 g of TSN, 6.62 g of MASN,and 2.03 g of DABA were polymerized and imidated in 153 g of PCP as asolvent at 190° C. for 29 hours to obtain a polyimide A solution havinga polymer concentration of 18 wt %. The polyimide A had anumber-averaged polymerization index N_(A) of 22 as measured by theabove-described GPC method.

In a separable flask, 21.18 g of s-BPDA and 20.25 g of TSN werepolymerized and imidated in 177 g of PCP as a solvent at 190° C. for0.25 hours to obtain a polyimide B solution having a polymerconcentration of 18 wt %. The polyimide B was found to have anumber-averaged polymerization index N_(B) of 4.5 as measured by theabove-described GPC method.

Eighty-eight grams of the polyimide A solution and 110 g of thepolyimide B solution were weighed out and mixed in a separable flask.The resulting multi-component polyimide blend solution was subjected tofurther polymerization and imidation at 190° C. for 29 hours to obtain amulti-component polyimide blend solution having a rotational viscosityof 1172 poise and a polymer concentration of 18 wt %. Thenumber-averaged polymerization index of the multi-component polyimidewas found to be 45 by the above-described GPC method.

An asymmetric membrane was prepared using the multi-component polyimideblend solution. The characteristics of the resulting asymmetric membranewere measured. The results obtained are shown in Table 3 below.

Example 12

In a separable flask, 23.10 g of 6FDA, 3.66 g of TSN, 6.62 g of MASN,and 2.03 g of DABA were polymerized and imidated in 153 g of PCP as asolvent at 190° C. for 0.5 hours to obtain a polyimide A solution havinga polymer concentration of 18 wt %. The polyimide A had anumber-averaged polymerization index N_(A) of 2.76 as measured by theabove-described GPC method.

In a separable flask, 21.18 g of s-BPDA and 20.25 g of TSN werepolymerized and imidated in 177 g of PCP as a solvent at 190° C. for 0.2hours to obtain a polyimide B solution having a polymer concentration of18 wt %. The polyimide B was found to have a number-averagedpolymerization index N_(B) of 3.1 as measured by the above-described GPCmethod.

Eighty-eight grams of the polyimide A solution and 110 g of thepolyimide B solution were weighed out and mixed in a separable flask.The resulting multi-component polyimide blend solution was subjected tofurther polymerization and imidation at 190° C. for 19 hours to obtain amulti-component polyimide blend solution having a rotational viscosityof 1618 poise and a polymer concentration of 18 wt %. Thenumber-averaged polymerization index of the multicomponent polyimide wasfound to be 78 by the above-described GPC method.

An asymmetric membrane was prepared using the multi-component polyimideblend solution. The characteristics of the resulting asymmetric membranewere measured. The results obtained are shown in Table 3 below.

Comparative Example 3

In a separable flask, 6.36 g of s-BPDA and 6.07 g of TSN werepolymerized and imidated in 171 g of PCP as a solvent at 190° C. for 5hours to obtain a polyimide B solution having a polymer concentration of6.4 wt %. The polyimide B was found to have a number-averagedpolymerization index N_(B) of 7.4 as measured by the above-described GPCmethod. To the polyimide solution were added 6.36 g of s-BPDA, 12.79 gof 6FDA, 8.10 g of TSN, 3.67 g of MASN, 1.12 g of DABA, and 20 g of PCPas a solvent. The resulting multi-component polyimide blend solution wassubjected to further polymerization and imidation at 190° C. for 23hours to obtain a multi-component polyimide blend solution having arotational viscosity of 1079 poise and a polymer concentration of 18 wt%. The number-averaged polymerization index of the multi-componentpolyimide was found to be 49 as a result of the above-described GPCmeasurement.

An asymmetric membrane was prepared using the multi-component polyimideblend solution. The characteristics of the resulting asymmetric membranewere measured. The results obtained are shown in Table 3 below.

In Comparative Example 3, the combination of N_(A) and N_(B) are out ofthe range satisfying equation 1 (i.e., in region A of FIG. 4). The filmhad Φ_(s)/f of 1.07 and a tensile elongation at break as low as 8%.

Example 13

In a separable flask, 12.36 g of s-BPDA and 11.35 g of TSN werepolymerized and imidated in 158 g of PCP as a solvent at 190° C. for 30hours to obtain a polyimide B solution having a polymer concentration of11.8 wt %. The polyimide B was found to have a number-averagedpolymerization index N_(B) of 75 as measured by the above-described GPCmethod. To the polyimide solution were added 12.44 g of 6FDA, 3.77 g ofMASN, 1.64 g of MPD, and 20 g of PCP as a solvent. The resultingmulti-component polyimide blend solution was subjected to furtherpolymerization and imidation at 190° C. for 6 hours to obtain amulti-component polyimide blend solution having a rotational viscosityof 1432 poise and a polymer concentration of 18 wt %. Thenumber-averaged polymerization index of the multi-component polyimidewas found to be 101 by the above-described GPC method.

An asymmetric membrane was prepared using the multi-component polyimideblend solution. The characteristics of the resulting asymmetric membranewere measured. The results obtained are shown in Table 3.

Comparative Example 4

In a separable flask, 21.32 g of 6FDA, 8.44 g of TSN, 2.81 g of DABAwere polymerized and imidated in 129 g of PCP as a solvent at 190° C.for 40 hours to obtain a polyimide A solution having a polymerconcentration of 19.3 wt %. The polyimide A had a number-averagedpolymerization index N_(A) of 26 as measured by the above-described GPCmethod.

In a separable flask, 27.41 g of s-BPDA, 22.22 g of TSN, and 1.80 g ofDADE were polymerized and imidated in 210 g of PCP as a solvent at 190°C. for 40 hours to obtain a polyimide B solution having a polymerconcentration of 18.7 wt %. The polyimide B was found to have anumber-averaged polymerization index N_(B) of 47 as measured by theabove-described GPC method.

Ninety grams of the polyimide A solution and 100 g of the polyimide Bsolution were weighed out and mixed in a separable flask. The resultingmulti-component polyimide blend solution was subjected to furtherpolymerization and imidation at 190° C. for 3 hours to obtain amulti-component polyimide blend solution having a rotational viscosityof 1711 poise and a polymer concentration of 19 wt %. Thenumber-averaged polymerization index of the multi-component polyimidewas found to be 52 by the above-described GPC method.

An asymmetric membrane was prepared using the multi-component polyimideblend solution. The characteristics of the resulting asymmetric membranewere measured. The results obtained are shown in Table 3.

In Comparative Example 4, the combination of N_(A) and N_(B) are out ofthe range satisfying equation 1 (i.e., in region B of FIG. 4). The filmhad Φ_(s)/f of 1.82. The ratio of a hydrogen gas permeation rate(H_(H2)) to a nitrogen gas permeation rate (P′_(N2)), i.e.,P′_(H2)/P′_(N2), was 11.

Comparative Example 5

In a separable flask, 26.65 g of 6FDA, 10.49 g of TSN, and 3.49 g ofDABA were polymerized and imidated in 161 g of PCP as a solvent at 190°C. for 40 hours to obtain a polyimide A solution having a polymerconcentration of 19.3 wt %. The polyimide A had a number-averagedpolymerization index N_(A) of 44 as measured by the above-described GPCmethod.

In a separable flask, 52.66 g of s-BPDA, 46.00 g of TSN, and 3.73 g ofDADE were polymerized and imidated in 419 g of PCP as a solvent at 190°C. for 25 hours to obtain a polyimide B solution having a polymerconcentration of 18.7 wt %. The polyimide B was found to have anumber-averaged polymerization index N_(B) of 66 as measured by theabove-described GPC method.

Ninety grams of the polyimide A solution and 100 g of the polyimide Bsolution were weighed out and mixed in a separable flask. The resultingmulti-component polyimide blend solution was stirred at 130° C. for 3hours to prepare a multicomponent polyimide blend solution having arotational viscosity of 2753 poise and a polymer concentration of 19 wt%. The number-averaged polymerization index of the multicomponentpolyimide was found to be 56 by the above-described GPC method.

An asymmetric membrane was prepared using the multi-component polyimideblend solution. The characteristics of the resulting asymmetric membranewere measured. The results obtained are shown in Table 3.

In Comparative Example 5, the combination of N_(A) and N_(B) are out ofthe range satisfying equation 1 (i.e., in region B of FIG. 4), and thetwo polyimide solutions were merely mixed with no substantial imidationreaction. The Φ_(s)/f value was 2.06, and the hydrogen gas permeationrate (P′_(H2)) to nitrogen gas permeation rate (P′_(N2)) ratio, i.e.,P′_(H2)/P′_(N2), was 3.

TABLE 3 Multi-component Polyimide Blend solution Number-Avg. TotalPolymerization Solution Polyimide A Polyimide B Monomer Degree ViscosityMonomer Monomer Composition After After (Polymer Component N_(A)Component N_(B) Step 1 Step 2 Concn.) Example 1 6FDA 12.44 g 0.5 s-BPDA74 s-BPDA 1.2 41 2046 poise (SY4-9) TSN 5.21 g 12.36 g 12.36 g (18 wt %)DABA 1.73 g TSN 11.35 g 6FDA 12.44 g (B/A = 1.085) (B/A = 0.985) TSN16.56 g DABA 1.73 g (B/A = 1.025) Example 2 6FDA 12.44 g 0.5 s-BPDA 75s-BPDA 1.2 41 1693 poise (SY4-25) TSN 4.17 g 12.36 g 12.36 g (18 wt %)MASN 3.77 g TSN 11.35 g 6FDA 12.44 g (B/A = 1.085) (B/A = 0.985) TSN15.52 g MASN 3.77 g (B/A = 1.025) Example 3 s-BPDA 6.36 g 31 s-BPDA 0.5s-BPDA 1.6 46 1246 poise (SY4-23) 6FDA 12.79 g 6.36 g 12.72 g (18 wt %)TSN 8.10 g TSN 6.07 g 6FDA 12.79 g MASN 3.67 g (B/A = 1.025) TSN 14.17 gDABA 1.12 g MASN 3.67 g (B/A = 1.025) DABA 1.12 g (B/A = 1.025) Example4 6FDA 12.44 g 0.5 s-BPDA 76 s-BPDA 1.2 45  911 poise (SY4-28) TSN 2.08g 12.36 g 12.36 g (18 wt %) MASN 3.77 g TSN 11.35 g 6FDA 12.44 g DABA1.16 g (B/A = 0.985) TSN 13.43 g (B/A = 1.085) MASN 3.77 g DABA 1.16 g(B/A = 1.025) Example 5 6FDA 12.79 g 0.5 s-BPDA 79 s-BPDA 1.2 73 1767poise (SY4-21) TSN 2.02 g 12.71 g 12.71 g (18 wt %) MASN 3.67 g TSN12.15 g 6FDA 12.79 g DABA 1.12 g (B/A = 1.025) TSN 14.17 g (B/A = 1.025)MASN 3.67 g DABA 1.12 g (B/A = 1.025) Example 6 s-BPDA 6.36 g 0.5 s-BPDA57 s-BPDA 0.7 50 1507 poise (SY4-22) 6FDA 12.79 g 6.36 g 12.72 g (18 wt%) TSN 8.10 g TSN 6.07 g 6FDA 12.79 g MASN 3.67 g (B/A = 1.025) TSN14.17 g DABA 1.12 g MASN 3.67 g (B/A = 1.025) DABA 1.12 g (B/A = 1.025)Example 7 6FDA 12.44 g 2.7 s-BPDA 0.5 s-BPDA 0.7 72 1265 poise (TY3-4-7)DABA 4.37 g 12.36 g 12.36 g (18 wt %) (B/A = 1.025) TSN 11.81 g 6FDA12.44 g (B/A = 1.025) TSN 11.81 g DABA 4.37 g (B/A = 1.025) Example 86FDA 12.44 g 2.1 s-BPDA 0.5 s-BPDA 0.7 78 1469 poise (TY3-6-7) DABA 4.37g 12.36 g 12.36 g (18 wt %) (B/A = 1.025) TSN 11.81 g 6FDA 12.44 g (B/A= 1.025) TSN 11.81 g DABA 4.37 g (B/A = 1.025) Example 9 6FDA 10.86 g4.9 s-BPDA 51 s-BPDA 10.7 62 2232 poise (SY5-16) TSN 1.72 g 10.79 g10.79 g (18 wt %) MASN 3.11 g TSN 10.31 g 6FDA 10.86 g DABA 0.95 g (B/A= 1.025) TSN 12.03 g (B/A = 1.025) MASN 3.11 g DABA 0.95 g (B/A = 1.025)Example 6FDA 10.86 g 4.9 s-BPDA 6.0 s-BPDA 5.51 57 1376 poise 10 TSN1.72 g 10.79 g 10.79 g (18 wt %) (SY5-17) MASN 3.11 g TSN 10.31 g 6FDA10.86 g DABA 0.95 g (B/A = 1.025) TSN 12.03 g (B/A = 1.025) MASN 3.11 gDABA 0.95 g (B/A = 1.025) Example 6FDA 10.86 g 22 s-BPDA 4.5 s-BPDA 6.6045 1172 poise 11 TSN 1.72 g 10.79 g 10.79 g (18 wt %) (SY5-18) MASN 3.11g TSN 10.31 g 6FDA 10.86 g DABA 0.95 g (B/A = 1.025) TSN 12.03 g (B/A =1.025) MASN 3.11 g DABA 0.95 g (B/A = 1.025) Example 6FDA 10.86 g 2.76s-BPDA 3.1 s-BPDA 2.97 78 1618 poise 12 TSN 1.72 g 10.79 g 10.79 g (18wt %) (SY5-19) MASN 3.11 g TSN 10.31 g 6FDA 10.86 g DABA 0.95 g (B/A =1.025) TSN 12.03 g (B/A = 1.025) MASN 3.11 g DABA 0.95 g (B/A = 1.025)Example 6FDA 12.44 g 0.5 s-BPDA 75 s-BPDA 1.24 101 1432 poise 13 MASN3.77 g 12.36 g 12.36 g (18 wt %) (SY5-21) MPD 1.64 g TSN 11.35 g 6FDA12.44 g (B/A = 1.085) (B/A = 0.985) TSN 11.35 g MASN 3.77 g DABA 1.64 g(B/A = 1.025) COMPARATIVE s-BPDA 12.71 g 45 1097 poise EXAMPLE 1 6FDA12.79 g (18 wt %) (SY4-27) TSN 16.20 g MASN 3.67 g (B/A = 1.025)COMPARATIVE s-BPDA 12.71 g 49 1190 poise EXAMPLE 2 6FDA 12.79 g (18 wt%) (SY4-19) TSN 14.17 g MASN 3.67 g DABA 1.12 g (B/A = 1.025)COMPARATIVE s-BPDA 6.36 g 0.5 s-BPDA 7.4 s-BPDA 12.72 g 0.7 49 1079poise EXAMPLE 3 6FDA12.79 g 6.36 g 6FDA 12.79 g (18 wt %) (SY5-14) TSN8.10 g TSN 6.07 g TSN 14.17 g MASN 3.67 (B/A = 1.025) MASN 3.67 g DABA1.12 g DABA 1.12 g (B/A = 1.025) (B/A = 1.025) COMPARATIVE 6FDA 12.22 g26 s-BPDA 47 s-BPDA 10.71 g 36 52 1711 poise EXAMPLE 4 TSN 4.84 g 10.71g 6FDA 12.22 g (19 wt %) (SY5-11) DABA 1.61 g TSN 8.68 g TSN 13.52 g(B/A = 1.025) DADE 0.70 DADE 0.70 g (B/A = 0.966) DABA 1.61 g (B/A =0.99) COMPARATIVE 6FDA 12.24 g 44 s-BPDA 66 s-BPDA 10.38 g 55 56 2753poise EXAMPLE 5 TSN 4.82 g 10.38 g 6FDA 12.24 g (19 wt %) (SY5-1) DABA1.60 g TSN 9.02 g TSN 13.84 g (B/A = 1.025) DADE 0.73 g DADE 0.73 g (B/A= 1.035) DABA 1.60 g (B/A = 1.029) Results of Evaluation of AsymmetricMembrane Tensile Elongation Gas Permeation Rate (cm³(STP)cm² · sec ·cmHg) at Break P′H2 × P′He × P′N2 × P′O2 × P′CO2 × P′CH4 × Φ_(s)/f (%)10⁻⁴ 10⁻⁴ 10⁻⁵ 10⁻⁵ 10⁻⁵ 10⁻⁵ Example 1 1.43 38 5.49 5.81 0.92 3.61 11.70.66 (SY4-9) Example 2 1.48 47 9.66 9.89 2.49 (SY4-25) Example 3 1.18 168.33 8.48 1.32 (SY4-23) Example 4 1.53 32 8.01 8.15 1.65 (SY4-28)Example 5 1.55 37 6.37 6.41 0.88 (SY4-21) Example 6 1.16 26 7.02 7.100.93 (SY4-22) Example 7 1.37 25 4.74 4.7 0.65 (TY3-4-7) Example 8 1.4815 60.5 6.1 1.03 (TY3-6-7) Example 9 1.69 33 8.59 8.76 1.67 (SY5-16)Example 1.33 42 5.54 5.54 0.83 10 (SY5-17) Example 1.58 25 6.92 7.0 0.9211 (SY5-18) Example 1.19 23 5.81 5.82 0.91 12 (SY5-19) Example 1.66 486.99 7.07 0.81 13 (SY5-21) COMPARATIVE 1.02 7 6.01 6.04 1.21 EXAMPLE 1(SY4-27) COMPARATIVE 1.04 7 8.10 8.24 1.11 EXAMPLE 2 (SY4-19)COMPARATIVE 1.07 8 7.4 7.52 1.00 EXAMPLE 3 (SY5-14) COMPARATIVE 1.82 731.57 1.34 1.48 EXAMPLE 4 (SY5-11) COMPARATIVE 2.06 36 1.42 1. 4.51EXAMPLE 5 (SY5-1) Note: B/A indicates molar ratio of diaminecomponent(s) to tetracarboxylic acid component B/A indicates molar ratioof diamine component(s) to tetracarboxylic acid component(s).

INDUSTRIAL APPLICABILITY

The present invention provides a polyimide asymmetric membrane having adense layer and a porous layer which is made of multi-componentpolyimide containing a fluorine-containing polyimide and has a properlycontrolled composition of the fluorine-containing polyimide in its denselayer.

The polyimide asymmetric membrane of the invention is suited for use asa practical high-performance gas separation membrane, with whichseparation between hydrogen gas and a hydrocarbon gas such as methane,separation between hydrogen and nitrogen, separation between helium andnitrogen, separation between carbonic acid gas and a hydrocarbon gassuch as methane, and separation between oxygen and nitrogen, and soforth can be accomplished advantageously.

1. A polyimide asymmetric membrane having a dense layer and a porouslayer, the membrane comprising a fluorine-containing polyimide and afluorine-free polyimide, the dense layer containing a higher fraction offluorine-containing polyimide, the ratio of the dense layer's fluorineatom concentration (Φ_(s)), measured by X-ray photoelectron spectroscopy(XPS), to the overall average fluorine atom concentration (f) of themembrane, Φ_(s)/f ranging from 1.1 to 1.8, wherein microphase-separationoccurs in the dense layer without being accompanied bymacrophase-separation.
 2. A gas separation membrane comprising thepolyimide asymmetric membrane of claim
 1. 3. The gas separation membraneaccording to claim 2 which is a hollow fiber membrane having a hydrogengas permeation rate (P′_(H2)) of 4.0×10⁻⁴ cm³(STP)/cm²·sec·cmHg orgreater, a hydrogen gas permeation rate (P′_(H2)) to nitrogen gaspermeation rate (P′_(N2)) ratio, P′_(H2)/P′_(N2), of 20 or greater, anda hollow fiber tensile elongation at break of 15% or more.
 4. The gasseparation membrane according to claim 2 which is a hollow fibermembrane having a helium gas permeation rate (P′_(He)) of 4.0×10⁻⁴cm³(STP)/cm²·sec·cmHg or greater, a helium gas permeation rate (P′_(He))to nitrogen gas permeation rate (P′_(N2)) ratio, P′_(He)/P′_(N2), of 20or greater, and a hollow fiber tensile elongation at break of 15% ormore.
 5. A method of selectively separating and recovering at least onekind of gas from a mixed gas, comprising feeding the mixed gas to a feedside of the gas separation membrane of claim 2 and selectively passingthe at least one kind of gas of the mixed gas through the gas separationmembrane to a permeate side of the gas separation membrane.